1. Field of the Invention
The present invention relates to an exposure apparatus which illuminates a reticle with illumination light from a light source and projects light from the reticle onto a substrate to expose the substrate to light, and a method of manufacturing a device using the exposure apparatus.
2. Description of the Related Art
Japanese Patent Publication No. 61-34252 describes an exposure apparatus with an arrangement of detecting the intensity of light coming from a substrate or going to the substrate by a photoelectric detector, obtaining the output pulses of a frequency corresponding to the detection value, counting the pulses, and closing a shutter when the number of counted pulses reaches a predetermined pulse count. In the shutter, one rotary disk is provided with a light-shielding portion and light-transmitting portion alternately. By rotating the shutter, the light-shielding state and light-transmitting state of illumination light are controlled. An error caused by an operation delay time of the shutter, that is, by dose (exposure amount) of the substrate during a period since a shutter closing signal is generated until the shutter closes completely, must be corrected. For this purpose, the pulses corresponding to the dose due to the shutter operation delay when opening the shutter are counted. The timing to generate the shutter closing signal is corrected considering the pulse count.
When performing exposure control with a small dose, with the arrangement described in Japanese Patent Publication No. 61-34252, the timing to close the shutter may sometimes not be in time. To prevent this, a method of closing the shutter without counting the pulses, or a method of counting the pulses as described above while the light intensity is low and closing the shutter when the number of counted pulses reaches a predetermined pulse count can be adopted. As the method of decreasing the light intensity, a method of moving the position of the light source along the optical axis, and a method of inserting a neutral density filter between the light source and substrate may be possible.
The method of closing the shutter without counting the pulses, however, requires a technique to guarantee that the wafer or the shot region is exposed with an appropriate dose. With the method of performing exposure control as described above upon reducing the light intensity, the throughput decreases.